Stabilization of polyester fibers with carbodiimide

ABSTRACT

The carboxyl end group concentration of polyethylene terephthalate fibers is lowered with a corresponding increase in hydrolytic stability by the surface application of a carbodiimide having the general formula:   WHERE X IS A RADICAL SELECTED FROM THE GROUP CONSISTING OF 2,4 TOLYLENE, 2,6 TOLYLENE AND MIXTURES OF THE ABOVE; AND N HAS AN AVERAGE VALUE OF ABOUT 1.

United States Patent 91 Batchelor, Jr.

[ 1 Dec. 3,1974

1 1 STABILIZATION OF POLYESTER FIBERS WITH CARBODIIMIDE [52] US. Cl.117/l38.8 F, 8/115.6, 57/153,

260/459 R [51] Int. Cl. C08g 51/60, D06m 13/38 [58] Field of Search117/138.8 F; 260/459 R, 260/551 CD; 8/115.6; 57/153 [56] ReferencesCited UNITED STATES PATENTS 2,853,473 9/1958 Campbell et a1 260/4593,193,522 7/1965 Neumann et al. 260/459 3,193,524 7/1965 Holtschmidt eta1. 260/459 3,296,190 1/1967 Reischl et a1. 260/459 Nolen 260/459 Wcimar117/l38.8

Primary Examiner-Herbert B. Guynn Attorney, Agent, or Firm-Thomas Y.Awalt, .lr.

[ 5 7 ABSTRACT The carboxyl end group concentration of polyethyleneterephthalate fibers is lowered with a corresponding increase inhydrolytic stability by the surface application of a carbodiimide havingthe general formula:

where x is a radical selected from the group consisting of 2,4 tolylene,2,6 tolylene and mixtures of the above; and n has an average value ofabout 1.

8 Claims, No Drawings STABILIZATION OF POLYESTER FIBERS WITHCARBODIIMIDE Syntheticlinear polyester filaments, yarns and cords areknown to show improved strength under hydrolytic conditions or elevatedtemperatures when, either by the use of certain additives to thepolymer, or chemical coatings on the polyester fiber, the carboxyl endgroup concentration is lowered.

One of the commonly employed methods of lowering the carboxyl level isby applying to the polyester fiber a surface coating which consistsessentially of a carbodiimide or polycarbodiimide. United States patentsrelating to the use of such carbodiimides or polycarbodiimides includeUS. Pat. Nos. 3,193,522, 3,193,523 and 3,193,524. I

It is an object of this invention to provide polyester fibers having alow carboxyl end group concentration with further improved hydrolyticand aminolytic stability.

It is yet another object of this invention to provide a process for thetreatment of polyester fibers so that much improved hydrolytic andaminolytic stability may be obtained.

Briefly, the objects of this invention are attained by applying a liquidcarbodiimide or a solution of the liquid carbodiimide reaction productof tolylene diisocyanate and phenyl isocyanate to the surface of thefiber by any convenient manner such as coating, spraying or dipping. Thereaction product sought from the tolylene diisocyanate and phenylisocyanate is a compound having the general formula:

where x is a radical selected from the group consisting of 2,4 tolylene,2,6 .tolylene and mixtures-of the above; and n has an average value ofabout 1.

l have discovered that superior hydrolytic and aminolytic stability inrubber tire reinforcements are obtained in the use of this particularcarbodiimide-as a stabilizing coating.

As usedherein the term rubber" is intended to be used in its usual andaccepted generic sense to include rubber substitutes, natural rubber,compounded rubber, synthetic rubber, and the like. The term carboxyllevel" is used to include both the un-ionized acid group, COOH and theionized group, COO Determination of the concentration of carboxyl groupsis made in accordance with the procedure described by Pohl in AnalyticalChemistry, Volume 26, page 1614, October, 1954, and is expressed inmilliequivalents per gram. The term relative viscosity refers to theratio of the viscosity of a percent solution of polyethyleneterephthalate in a mixture of 10 parts of phenol and 7 parts of 2,4,6trichlorophenol (by weight) to the viscosity of thephenol-trichlorophenol mixture, measured in the same units at 25 C.Specific viscosity (Nsp), as re flected in the data contained herein,was obtained according to procedures well known in the art, employing0.5 percent of the polymer in a 2:1 solution of phenol/-trichlorophenol.

The term intrinsic viscosity refers to the limit of the fraction r/c asc approaches 0, where r is the relative viscosity as measured above,except that it is measured at several different concentrations in orderto extrapolate to zero concentration.

Polyester fibers are those fibers composed. of at least percent byweight of an ester of a dihydric alcohol and terephthalic acid. Thefibers include copolyesters of modifications of these polyesters andcopolyesters.

Those polyesters and co-polyesters specifically useful in the instantinvention are those resulting from heating one or more of the glycols ofthe series I-IO(CH ),,OI-I, in which n is an integer from 2 to 10, withone or more dicarboxylic acids. Among the dicarboxylic acids useful'inthe present invention are terephthalic acid, isophthalic acid, sebacicacid, adipic acid, p-carboxyphenylacetic acid, succinic acid,p,p'-dicarboxyphenylthiocarbanilide, P4 '-dicarboxydiphenylsulfone,p-carboxyphenoxyacetic acid, p-carboxyphenoxypropionic acid,p-carboxyphenoxybutyric acid, p-carboxyphenoxyvaleric acid,p-carboxyphenoxyhexanoic acid, p-carboxyphenoxyheptanoic acid,p,p-dic,arboxydiphenylmethane, p,p'-dicarboxydiphenylethane, p,p'-dicarboxydiphenylpropane, p,p'-dicarboxydiphenylbutane,p,p'-dicarboxydiphenylpentane, p,p'-dicarboxydiphenylhexane,p,p'-dicarboxydiphenylheptane, p,p'-dicarboxydiphenyloctane,p,p'-dicarboxydiphenoxyethane, p,p'-dicarboxydiphenoxypropane, p,p-'-dicarboxydiphenoxybutane, p,p-dicarboxydiphenoxypentane,p,p'-dicarboxydiphenoxyhexane, 3- alkyl-4-(beta-carboxyethoxy) benzoicacid, oxalic acid, glutaric acid, pimelic acid, suberic acid, azelaicacid and the dioxy acids of ethylene dioxide having the general formulawherein n is an integer from I to 4, and the like. Examples of theglycols which may be employed in practicing the instant invention areethylene glycol, trimethylene glycol, tetramethylene glycol,decamethylene glycol, cyclohexane dimethanol and the like. Polyethyleneterephthalate, however, is preferred because of the ready availabilityof terephthalic acid and ethylene glycol, from which it is made.

In order to obtain superior resistance to fatigue and thermaldegradation, it is essential that the carboxyl concentration not beexcessive. It has long been known that an excessively high concentrationof free carboxyl groups results in fibers which exhibit an excessiveloss of strength when used as reinforcements of rubber and when exposed.to temperatures in. excess of about 1 10 C. over prolonged period oftime.

It is also essential that the fibers of this invention be prepared frompolyethylene terephthalate polymers having a low concentration ofaliphatic ether groups. As is well known in the art, the concentrationsof ether groups should be no more than about 3 mole percent. At a higherlevel, the melting point of the polymer will be lower, and the rate ofthermal. degradation will be higher. Preferably, the concentration ofether groups should be less than about 2.5 mole percent. This may beachieved by purity of raw materials and/or by the use of inhibiters tothe formation of diethylene glycol, as is well known in the art.

Also, as is well known in the art, it isessential that filaments ofpolyethylene terephthalate used in tire yarns and the like be spun froma polymer having a relative viscosity of at least about 60. It ispreferable that filaments designed for use under severe conditions havea relative viscosity of at least about 80.

High viscosity or high molecular weight linear polyesters can beproduced on a commercial scale by an ester interchange process. Forexample, in the production of highly polymeric linear polyethyleneterephthalate, the dimethyl ester of terephthalic acid is heated with anexcess of ethylene glycol in the presence of an ester interchangecatalyst to produce, the bisglycol ester of terephthalic acid, withmethyl alcohol and excess ethylene glycol being distilled off. Theproduct is then polymerized by condensation reaction with theelimination of ethylene glycol by heating the product with a catalyst atelevated temperatures and under reduced pressures until a high molecularweight product is formed.

High molecular weight linear polyesters have also been produced by thedirect esterification process. For example, in the production of highlypolymeric linear polyethylene terephthalate, terephthalic acid may beheated with ethylene glycol to form the diglycol ester and low polymerthereof which can then be polymer ized by heating in the presence of acatalyst under reduced pressures to form a high molecular weightproduct.

In order to provide a desired tenacity of at least about 6 grams perdenier, filaments are drawn about 3-5 times their original length as iswell known in the art. The drawing steps may be accomplished before orafter treatments used to reduce free carboxyl groups when the polymerhas an excessive concentration thereof.

To effect adhesion between the linear terephthalate polyester cords anda rubber, it is necessary to apply an adhesive coating to the cords. Anyadhesive mixture may be used if it gives the desired level of adhesionbetween cord and rubber. For example, the cords may be coated with anorganic solvent solution of a rubber and polyisocyanate is described inUS. Pat. No. 2,415,839. Alternatively, the cords may be coated with anaqueous dispersion of a blocked isocyanate and a latex as described byThompson et al, Adhesive Age, Vol. ll, page 30, February 1959.

In order to obtain top level performance from reinforced rubberarticles, it is a general practice to first combine yarn ends into acord of the desired size and twist, and then apply heat to the cordunder tension in order to stabilize the cord properties and therebyreduce both shrinkage and growth, as well as obtaining higher strength.Advantageously the adhesive mixture may be applied to the cordimmediately prior to the hot tensioning operation so that the adhesiveis cured onto the cord during the heating step. Alternatively, theadhesive may be applied and dried onto the cord in a separate operation.

The carbodiimide stabilizer of this invention is prepared bycondensation of about 1.0 mole of tolylene diisocyanate and from about2.0-2.4 moles of phenyl isocyanate with about 0.005 moles of3-methyl-l-ethyl- 3-phosphalene oxide as a catalyst. The reactionproducts may be represented by the general structure.

where the mixture is n 0, 30 percent by weight n l, 35 percent by weightn 1, 35 percent by weight The structure where n l is the desired productand is the most effective component of the mixture. Thetolylenediisocyanate portion may be of the 2,4 configuration or amixture of 2,4 and 2,6 isomers.

EXAMPLES To the fiber surface of polyethylene terephthalate fibersprepared generally as described above, was applied a 2 percent (based onthe weight of the fiber) coating of the carbodiimide of this invention.Fibers were then heated at C. for 3 minutes. The treatment lowered thecarboxyl end group concentration from 32 to Zueq/gram carboxyls. Treatedsamples were compared with untreated samples by measurement of breakingstrength retained after hydrolysis and aminolysis. Samples weresubjected to hydrolysis according to the Goodyear Sealed Tube Testhereafter described.

Samples were subjected to aminolysis by Goodyear Test, F .D. No. 9-7entitled Aminolytic Degradation of Polyester hereafter described.

Results were as follows:

In order to obtain comparative data of the effect of other commerciallyavailable carbodiimides on conventional polyester tire yarn, samples ofthe tire yarn were prepared from bundles of 1,000 denier/200 filamentpolyethylene terephthalate fibers surface treated with the carbodiimidesindicated with subsequent heating for 3 minutes at 185 C. A comparisonbetween the performance of other commercially available carbodiimides asshown in Table ll demonstrates the superiority of the carbodiimide ofthis invention in terms of hydrolytic stability as measured by thestandard tests described below.

Table ll shows the additive treatment along with a 3 minute heating stepall of which was accomplished before elastic dipping. Curing of thecarbodiimide may, however, be effectively postponed until after elasticdipping.

TABLE ll Comparison of Various Carbudiimides As Surface Additives GSTTCS.

Additive "COOH Y+RSTT (JSTT Surface Additive on Yarn cq/gm C.S. CS.

Diphenyl Carhodiimide -w 22.6 11.4 6,7 Dicyclohcxyl Carhodiimide -l 29.6I43 9.2 Tolylcncdiisocyanute -l l3.0 9.3 6.3

Carbodiimide (poly) TABLE 11 Continued Comparison of VariousCarbodiimidcs As Surface Additives Additive COOH GSTT Y+RSTT GSTTSurface Additive on Yarn eq/gm C.S. CS. C.S.

Phcnyl lsocyanalc/Tolylcnc- 9.8 5.7 5.5

diisocy'ainatcifi carbodiimide Control (first series) 31.3 16.4 12.341.2

Commercially available ""2 30.3 14.1 14.6 28.5

carbodiimide Commercially available ""2 31.7 22.) 10.0 34.5

poly-carbodiimide Phcnyl lsocyanzltc/lolylcnc- "2 3.7 13.2 12.4 19.3diisocyanatc-2.4

carbodiimide Control (second series) 32.4 29.4 10.5 41.2

"Applicd by dipping 1300/3 cord into 2% solutions and heating 185 C./3min.

Cords heated 3 min. at 185 C. "Solutions 'mctercd on, dried at 100 C.

Abbreviations GSTT Goodyear Scaled Tube Test Y+RSTT Yarn plus RubberScaled Tube Test Y in RSTT Yarn in Rubber Sealed Tube Test GS. No. ofChain Sciss'ions Goodyear Sealed Tube Test (GSTT) Modified A sealed tubetest similar to Goodyear Test FD No. 9-3 was used to measure thehydrolytic stability of both greige and treated cords. A 1.5 gram sampleof cord was placed in a 50 ml pyrex pressure tube with a crown cap top.After conditioning the sample, tube, cap and neoprene rubber sealer for24 hours at 70 F. and 65 percent R.H., the tube was sealed while in thisenviron-. ment. The sealed tube was then placed in an oven at 149 C. for48 hours. After about 36 hours the sealed tube was opened and thebreaking strength of the cord measured. Ten breaks were made on the cordin each tube and two tubes were run on each cord sample. Ten breaks weremade on a sample of the cord not exposed to the sealed tube test toobtain the initial breaking strength. The following relationship wasused to calculate per cent breaking strength retention:

Percent BSR Av. Final Breaking Strength/Av. lnitial Breaking Strength X100 In addition to strength retention, the number of chain scissionsoccurring during the sealed tube test was calculated from the initialand final solution viscosities of the cords using the followingequation:

N0. Scissions in p:- moles/g =(l/T/1nF I/IT/lnl) X 10 where K'lnF andMn] are the final and initial number average molecular weights. Therelationship between number average and weight average molecular weightwas assumed to be Yarn in Rubber Sealed Tube Test (Y inRSTT) An aluminumstrip 1/ 16 inch X /4 inch X 5 inches was prepared with a notch in eachend. A sample of tire yarn or cord of 0.5' gm was wound lengthwise aboutthis strip. The composite is sandwiched between two layers of rubber andcured such that the cord and aluminum strip are completely embedded inthe rubber strip. The rubber was cured for 30 minutes, starting with themold at C., under 5 tons of pressure at 160 C. The cured strip measures5/16 inch X 6 inch X 6 inches and weighs approximately 13 grams. Thisstrip was placed in a 50 m1 Pyrex pressure tube and conditioned 24 hoursat F. and 65 percent RH, along with a neoprene seal and aCrown bottlecap. The tube was then capped while in the same atmosphere and thenheated 8 hours at C. After cooling to room temperature, the cord wasstripped out of the rubber and then submitted for Nsp measurements. Carewas taken to remove as much rubber as possible. The number of chainscissions" is calculated from viscosity data using the followingequation:

No. scissions (l/M, n X 106 I I (I it, is assumed to be 1.9 M Yarn plusRubber Sealed Tube Test (Y RSTT) Strips of rubber were cured in a moldfor 25 minutes, starting with a preheated mold, at C. and under 5 tonsof pressure. The mold yielded a pad of rubber 5/16 inch X 6 inches X 7inches which was cut into strips 5/16 inch X inch X 6 inches. Samples ofcord, or yarn, weighing 0.5 gms were wound tightly around the rubber andtied. This strip was placed inside a 50 m1 Pyrex pressure tube andconditioned, along with a neoprene seal and bottle cap, at 70 F. and 65percent RH. for 24 hours. The bottle was capped in this atmosphere andthen heated for 8 hours at 150 C. in a forced air oven; After cooling toroom temperature, the cord was removed and submitted for Nspmeasurements. The number of chain scissions was calculated as in thepreceding test.

Aminolytic degradation of yarn samples was determined by exposing thesamples in a sample tube at 150 C. to a stream of ammonia at a flow rateof 30-40 cc/min., for a period of 3 hours; and then testing for breakingstrength.

A unique feature of polyethylene terephthalate yarns treated with thecarbodiimide of this invention is its relatively slow rate of reactionwith water. While reactivity with water is reported to be a significantproblem with some classes of carbodiimides, yarns coated with 2 percentof the carbodiimide described above, and

without a precuring step, were heated for 24 and 48 hours at 80 C. and100 percent relative humidity. The reaction between the carboxyl endgroups of the polyester and the carbodiimide is apparently much fasterthan the attack of water on the carbodiimide. The carboxyl level in eachcase was reduced to less than 1 ueq/gram during this lowtemperature/high humidity heating step.

I claim:

1. Filaments composed of at least 85 percent by weight of an ester ofethylene glycol and terephthalate acid having an aliphatic etherconcentration of -3 mole percent and surface coated with about %2percent, based on the weight of the filaments of a compound having thegeneral structural formula:

where x is a radical selected from the group consisting of 2, 4tolylene, 2, 6 tolylene, and mixtures of the above; and n has an averagevalue of about 1.

2. The filaments of claim 1 wherein said coating is present in theamount of about 2' percent based on the weight of the filament.

3. Yarns comprised of the filaments of claim 1.

4. Tire cords comprised of the filaments of claim 1.

5. The process of stabilizing filaments composed of at least percent byweight of an ester of ethylene glycol and terephthalate acid having analiphatic ether content of 0-3 mole percent comprising contacting saidfilaments with a solution of a compound having the general formula:

where x is a radical selected from the group consisting of 2, 4tolylene, 2, 6 tolylene, and mixtures of the above; and n has an averagevalue of about 1, so as to provide a coating on the filaments of fromabout re-2 percent, based on the weight of the filaments; whereby thecarboxyl end group concentration of said ester is substantially reduced.

6. The process of claim 5 wherein the filaments are thereafter heated ata temperature of about C for about 3 minutes.

7. The process of claim 5 wherein said filaments are in the form ofyarn.

8. The process of claim 5 wherein the filaments are in the form of tirecords.

1. FILAMENTS COMPOSED OF AT LEAST 85 PERCENT BY WEIGHT OF AN ESTER OFETHYLENE GLYCOL AND TEREPHTHALATE ACID HAVING AN ALIPHATIC ETHERCONCENTRATION OF 0-3 MOLE PERCENT AND SURFACE COATED WITH ABOUT 1/2-2PERCENT, BASED ON TE WEIGHT OF THE FILAMENTS OF A COMPOUND HAVING THEGENERAL STRUCTURAL FORMULA:
 2. The filaments of claim 1 wherein saidcoating is present in the amount of about 2 percent based on the weightof the filament.
 3. Yarns comprised of the filaments of claim
 1. 4. Tirecords comprised of the filaments of claim
 1. 5. The process ofstabilizing filaments composed of at least 85 percent by weight of anester of ethylene glycol and terephthalate acid having an aliphaticether content of 0-3 mole percent comprising contacting said filamentswith a solution of a compound having the general formula:
 6. The processof claim 5 wherein the filaments are thereafter heated at a temperatureof about 185* C for about 3 minutes.
 7. The process of claim 5 whereinsaid filaments are in the form of yarn.
 8. The process of claim 5wherein the filaments are in the form of tire cords.